The present invention relates to an inspection method and an inspection device for an active matrix substrate used for an organic EL display device and the like, and an inspection program used therefor and an information storage medium.
In recent years, a display device in which self-emitting organic EL elements are arranged in the shape of a matrix array has been extensively developed.
Defect inspection of this type of organic EL display device is carried out before shipment from the factory after forming an organic EL film between an active matrix substrate and a common substrate and assembling all peripheral parts.
A technique of inspecting a display screen by visual inspection by driving the organic EL display device has been known. However, in the case of visual inspection, variation of inspection accuracy easily occurs depending on the condition of the inspector or individual variation. Moreover, it is impossible to determine whether the display defect is caused by an interconnect defect or whether the defect pixel is a dark spot. Furthermore, it is impossible to determine whether luminance nonuniformity of the screen is caused by a defect in an organic EL film, a defect in a drive section, or current leakage from interconnects. Therefore, it is necessary to check the cause of defects for a display device which is determined to be defective after inspection. However, since the display device is manufactured in a plurality of manufacturing steps, the cause of defects may be superimposed between each step. Therefore, data on occurrence of defects cannot be rapidly fed back to the manufacturing steps.
There has been proposed automation of inspection for an organic EL display device (Japanese Patent Application Laid-open No. 10-321367 and Japanese Patent Application Laid-open No. 2000-348861). Japanese Patent Application Laid-open No. 10-321367 discloses a method of evaluating organic EL elements by measuring leakage current flowing through the organic EL elements when applying a reverse bias voltage. In this method, it is necessary to apply a reverse bias voltage to the organic EL elements while suspending display drive which causes constant current to flow through the organic EL elements. Japanese Patent Application Laid-open No. 2000-348861 points out the possibility in which true drive characteristics may not be evaluated by the method disclosed in Japanese Patent Application Laid-open No. 10-321367 due to influence of drive interruption, and proposes inspecting organic EL elements by superimposing an inspection signal during driving in which a forward bias voltage is applied to the organic EL elements. In this method, the organic EL elements are evaluated based on changes in drive voltage and drive current when superimposing the inspection signal.
However, since the inspection is performed in a finished product state in these applications, time and materials used to manufacture the product are lost if defects are detected.
The present invention may provide an inspection method and an inspection device for an active matrix substrate capable of inspecting dot defects, line defects, or luminance defects in the stage of the active matrix substrate, and an inspection program used therefor and an information storage medium.
An inspection method for an active matrix substrate according to a first aspect of the present invention comprises:
a first step of providing an active matrix substrate including a plurality of signal lines, a plurality of scanning lines, a plurality of voltage supply lines, and a plurality of pixels, each of the plurality of pixels being connected with one of the signal lines, one of the scanning lines, and one of the voltage supply lines, each of the plurality of pixels including a pixel select transistor connected with the one signal line and the one scanning line and an operating transistor, a gate of the operating transistor being connected with the pixel select transistor, one of a source and a drain of the operating transistor being connected with the one voltage supply line, and the other of the source and the drain of the operating transistor being in an open state;
a second step of charging a parasitic capacitor between the gate of the operating transistor and the one voltage supply line by supplying a potential from an inspection device;
a third step of measuring discharge current by using the inspection device when discharging the parasitic capacitor; and
a fourth step of determining whether or not a defect exists in each of the plurality of pixels by using the inspection device based on a value of the discharge current.
In the first aspect of the present invention, a voltage is applied between the gate of the operating transistor and the one voltage supply line. This enables the parasitic capacitor between the gate of the operating transistor and the one voltage supply line to be charged even if the source or the drain which is not connected with the one voltage supply line is in an open state. The parasitic capacitor cannot be charged if a defect, such as a breakage of the one voltage supply line or a breakage between the gate of the operating transistor and the one voltage supply line, exists. Therefore, the above defect can be detected by charging the parasitic capacitor and monitoring current when discharging the parasitic capacitor. Since a voltage cannot be normally applied between the gate of the operating transistor and the one voltage supply line in the case where the voltage supply line and the like are short-circuited, the short-circuit can be detected as a defect by monitoring current during discharging. Moreover, a luminance nonuniformity defect caused by a difference in the parasitic capacitors between the pixels can also be detected.
Each of the plurality of pixels may further include a storage capacitor connected with the gate of the operating transistor. In this case, an influence of the storage capacitor may be canceled in the second step and the third step. This is because current must be measured depending only on the parasitic capacitor. The influence of the storage capacitor may be canceled by setting a potential difference between opposite ends of the storage capacitor to be substantially the same in the second step and the third step. This prevents the storage capacitor from being charged and discharged.
A range of capacitance values of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line may have a high-saturation region, a low-saturation region, and a transition region between the high-saturation region and the low-saturation region in which a capacitance value changes depending on an applied voltage. In this case, a voltage may be applied between the gate of the operating transistor and the one voltage supply line in at least one of the second step and the third step, so that a capacitance value of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line is within the high-saturation region.
The amount of charge to be stored in the parasitic capacitor is increased as the capacitance value of the parasitic capacitor is increased during charging, whereby a large amount of discharge current can be obtained. Even if the capacitance value of the parasitic capacitor Cdgo is small during charging, since current flows until the parasitic capacitor is in the equilibrium state if the capacitance value of the parasitic capacitor Cdgo is large during discharging, whereby a large amount of discharge current can also be obtained. This enables a signal/noise ratio (S/N) of monitoring current to be secured sufficiently.
In the first aspect of the present invention, a set of steps consisting of the second to fourth steps may be performed a plurality of times while changing a voltage applied between the gate of the operating transistor and the one voltage supply line.
Luminance nonuniformity of the pixels may be caused by characteristics of the operating transistor (for example, variation of voltage-dependent characteristics of the parasitic capacitor between the gate and the drain). There may be a case where luminance nonuniformity is not observed by visual inspection in an early stage even if the characteristics of the transistors vary. However, a luminance nonuniformity defect occurs with the elapse of time in such a case. The xe2x80x9cluminance nonuniformityxe2x80x9d used herein refers to xe2x80x9cdistributions of transistors having different characteristicsxe2x80x9d.
There may be a case where voltage-dependent variation of the parasitic capacitors cannot be detected by setting a voltage in the high-saturation region. Therefore, it is desirable to apply voltages at a plurality of points especially in the transition region and allow the parasitic capacitors to be charged and discharged at each of the points in order to measure voltage-dependent variation of the parasitic capacitors.
In this case, a luminance nonuniformity defect may be detected by measuring discharge current at a point at which the capacitance value of the parasitic capacitor is in the high-saturation region in the initial performance of the set, and measuring discharge current at a point at which the capacitance value of the parasitic capacitor is in the transition region after the initial performance. In this case, a pixel which is determined to be defective in the initial performance may not be determined after the initial performance. It is ideal to perform the charging step, the sensing step, and the determining step after the initial performance only for the pixels determined to be normal in the initial performance. However, in the case where the pixels are sequentially driven according to a fixed procedure, only the determining step for the defective pixel may be omitted.
An inspection method for an active matrix substrate according to a second aspect of the present invention comprises:
a first step of providing an active matrix substrate including a plurality of signal lines, a plurality of scanning lines, a plurality of voltage supply lines, and a plurality of pixels, each of the plurality of pixels being connected with one of the signal lines, one of the scanning lines, and one of the voltage supply lines, each of the plurality of pixels including a pixel select transistor connected with the one signal line and the one scanning line, an operating transistor, and a storage capacitor, a gate of the operating transistor being connected with the storage capacitor and the pixel select transistor, one of a source and a drain of the operating transistor being connected with the one voltage supply line, and the other of the source and the drain of the operating transistor being in an open state;
a second step of charging the storage capacitor by supplying a potential from an inspection device;
a third step of measuring discharge current from the storage capacitor by using the inspection device when discharging the storage capacitor; and
a fourth step of determining whether or not a defect exists in each of the plurality of pixels by using the inspection device based on a value of the discharge current,
wherein an influence of a parasitic capacitor between the gate of the operating transistor and the one voltage supply line is canceled in the second step and the third step.
According to the second aspect of the present invention, the storage capacitor can be charged and discharged in a state in which the influence of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line is canceled. In this case, since current is not discharged normally if a defect exists in the charge/discharge path to the storage capacitor, such as the case where a defect exists in the pixel select transistor, a pixel defect can be detected. In this case, since the influence of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line is canceled, discharge current can be measured without consideration of the presence of the operating transistor. Therefore, if a pixel is determined to be defective by the inspection method of the second aspect of the present invention, it is determined a the pixel defect exists in a pixel area other than the operating transistor. Therefore, whether or not a defect is caused by the operating transistor can be determined by performing the inspection method according to the second aspect of the present invention for a pixel which is determined to be defective by the inspection method according to the first aspect of the present invention. In the case where the pixels are sequentially driven according to a fixed procedure, only the determining step in the inspection method according to the second aspect of the present invention may be omitted for pixels which are determined to be normal by the inspection method according to the first aspect of the present invention.
The influence of the parasitic capacitor of the operating transistor may be canceled by setting a potential difference between the gate of the operating transistor and the one voltage supply line to be substantially the same in the second step and the third step. A range of capacitance values of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line may have a high-saturation region, a low-saturation region, and a transition region between the high-saturation region and the low-saturation region in which a capacitance value changes depending on an applied voltage. Therefore, a voltage may be applied between the gate of the operating transistor and the one voltage supply line in the second step and the third step, so that a capacitance value of the parasitic capacitor between the gate of the operating transistor and the one voltage supply line is within the low-saturation region. This enables the parasitic capacitor to be substantially ignored.
The sequential pixel drive performed in the first aspect and the second aspect of the present invention is the same as a display drive method for an active matrix display. According to this display drive method, a charge operation may be performed for pixels arranged in line among the plurality of pixels by sequentially setting the scanning lines at an active potential in the second step (first frame), the pixels arranged in line being connected with each of the scanning lines, and when all the plurality of pixels have been charged, a discharge operation may be performed for the pixels arranged in line connected with each of the scanning lines by sequentially setting the scanning lines at the active potential in the third step (second frame). In the case of an interlace driving, the charge operation is performed for all the pixels in odd-numbered and even-numbered rows by using first and second fields, and the discharge operation is performed for all the pixels by using third and fourth fields.
The pixels arranged in line may be driven by a line-at-a-time scanning or a point-at-a-time scanning. It is desirable to employ the point-at-a-time scanning since discharge current in the third step is obtained for each pixel instead of for each line as in the line-at-a-time scanning. In the point-at-a-time scanning, the pixels arranged in line may be driven by a point-at-a-time scanning by sequentially connecting the signal lines connected with the pixels arranged in line with the inspection device in the second step and the third step.
An object of inspection may be an active matrix substrate comprising a vertical driver circuit which selectively drives the scanning lines and a horizontal driver circuit which selectively drives the signal lines. In this case, the pixels are driven in the second step and the third step based on functions of the vertical driver circuit and the horizontal driver circuit.
An inspection device for an active matrix substrate according to a third aspect of the present invention inspects an active matrix substrate including a plurality of signal lines, a plurality of scanning lines, a plurality of voltage supply lines, and a plurality of pixels, each of the plurality of pixels being connected with one of the signal lines, one of the scanning lines, and one of the voltage supply lines, each of the plurality of pixels including a pixel select transistor connected with the one signal line and the one scanning line and an operating transistor, a gate of the operating transistor being connected with the pixel select transistor, one of a source and a drain of the operating transistor being connected with the one voltage supply line, and the other of the source and the drain of the operating transistor being in an open state, the inspection device comprising:
an inspection potential generating circuit which generates inspection potentials supplied to the scanning lines and the voltage supply lines;
a charge-sense circuit connected with the signal lines;
a timing signal generating circuit which generates a timing signal for driving the scanning lines, the signal lines, and the voltage supply lines; and
a determining circuit which determines whether or not a defect exists in each of the plurality of pixels based on an output from the charge-sense circuit,
wherein the inspection potential generating circuit and the charge-sense circuit supply potentials to charge a parasitic capacitor between the gate of the operating transistor and the one voltage supply line in a charging period, supply potentials to discharge the parasitic capacitor in a sensing period, and measure discharge current from the parasitic capacitor by using the charge-sense circuit in the sensing period.
An inspection device according to a fourth aspect of the present invention performs the inspection method according to the second aspect of the present invention by using the same hardware as the inspection device according to the third aspect of the present invention.
The inspection method according to the first or second aspect of the present invention can be appropriately performed by using the inspection device according to the third or fourth aspect of the present invention.
An inspection program according to a fifth or sixth aspect of the present invention makes a computer to perform a procedure for the inspection method according to the first or second aspect of the present invention.
A computer-readable information storage medium according to a seventh aspect of the present invention stores the inspection program according to the fifth or sixth aspect of the present invention.